add power function to APInt (#122788)

I am trying to calculate power function for APFloat, APInt to constant
fold vector reductions: https://github.com/llvm/llvm-project/pull/122450
I need this utility to fold N `mul`s into power.

---------

Co-authored-by: ImanHosseini <imanhosseini.17@gmail.com>
Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index ea8295f..38cf485 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -3108,3 +3108,21 @@ APInt APIntOps::mulhu(const APInt &C1, const APInt &C2) {
   APInt C2Ext = C2.zext(FullWidth);
   return (C1Ext * C2Ext).extractBits(C1.getBitWidth(), C1.getBitWidth());
 }
+
+APInt APIntOps::pow(const APInt &X, int64_t N) {
+  assert(N >= 0 && "negative exponents not supported.");
+  APInt Acc = APInt(X.getBitWidth(), 1);
+  if (N == 0)
+    return Acc;
+  APInt Base = X;
+  int64_t RemainingExponent = N;
+  while (RemainingExponent > 0) {
+    while (RemainingExponent % 2 == 0) {
+      Base *= Base;
+      RemainingExponent /= 2;
+    }
+    --RemainingExponent;
+    Acc *= Base;
+  }
+  return Acc;
+};